The invention is related to a wholly air-controlled impact mechanism for creating more compact, reliable, economical and powerful pneumatic devices, such as a pneumatic wrench.
The pneumatic wrench, driven by an air motor, is an efficient tool for mounting and dismounting bolts and nuts. Various mechanisms have been adopted by manufacturers. For decades, inventors and entrepreneurs have made great efforts to improve the performance of pneumatic wrenches. The energy-accumulating pneumatic wrench attracts the most attention. An air motor drives a flying hammer to a high speed. Subsequently, an impact pin stretches out from the hammer and imposes an impact torque on the anvil shaft. The higher the speed of the hammer, the larger the impact torque. Reliable and precise control of the motion of the impact pin is the key to unlock the full energy accumulated in the flying hammer.
FIG. 1 shows the action principle of a typical traditional design of impact mechanism for energy-accumulating pneumatic wrench. A brief description of its construction is as follows:
An eccentric pilot valve (b) is fitted in the cavity of a flywheel (a) and may slide along the radial direction. The pilot valve (b) is held in its retracted position by a spring (m) and a draw bar (n). An impulsive time-delay trigger is disposed in another cavity of the flywheel (a). It consists of a small spring (h), a trigger pin (l), a plunger (k) and an end cam (j) mounted on the anvil shaft.
During energy accumulation phase, the trigger pin (l) locks the pilot valve (b) at its retracted position. The flywheel (a) rotates relative to the anvil shaft. The plunger (k) and trigger pin (l) move up and down following the profile of the end cam. The cam profile pushes the trigger pin (l) into the body of pilot valve (b) with each rotation, during which time, the pilot valve (b) is unlocked and can potentially move outward along the radius.
There is an annular plenum (5) around the cylindrical surface of the pilot valve (b). The annular plenum (5) controls the direction of controlling air. When the pilot valve (b) rests at its retracted position, the controlling air from air inlet passage (3), through annular plenum (5) and retracting passage (6), comes into the lower plenum (16) of the impact pin (c), causing the impact pin (c) to rest at its retracted position. Refer to FIG. 1A.
When the flywheel (a) rotates with sufficiently high speed, the centrifugal force of the pilot valve (b) becomes large enough to overcome the pull of the spring (m). The pilot valve (b), when prompted by the impulsive trigger mechanism, begins to move outwards until limited by a stopper (g). The outward movement of the pilot valve (b) connects the air inlet passage (3) with the stretching passage (7) through the annular plenum (5). The controlling air from the inlet passage (3) travels through the annular plenum (5) and the stretching air passage (7), and enters into the upper plenum (15) of the impact pin (c), causing the impact pin (c) to stretch out from the flywheel (a). The impact pin (c) imposes an impact torque on the anvil shaft, tightening or loosening the nut. Refer to FIG. 1B.
The speed of flywheel (a) is decreased to zero upon the impact. The centrifugal force disappears, and the pilot valve (b) is pulled back to the cavity by the spring (m). The inward movement of the pilot value (b) switches the controlling air to the lower plenum (16) of the impact pin (c) through the retracting air passage (6), causing the impact pin (c) to retreat into the flywheel (a), as shown in FIG. 1A. The system is then ready for the next cycle of energy-accumulating and impacting.
The above described energy accumulating and impacting system has been applied in commercial pneumatic wrenches. However, it has a number of drawbacks. Due to the restraints of the spring (m), the pilot valve (b) cannot move rapidly enough to quickly drive the impact pin (c) into its fully stretched position, causing a series of “sliding” phenomenon, or so called “double hits”, which may result in parts damage.
The impulsive trigger mechanism improves the rapidity of outward movement of pilot valve (b). But, its effect is uncertain and unreliable due to its dependency on the initial position of the end cam (j), i.e. the initial position of the anvil shaft relative to the flywheel (a).
Furthermore, various parts of the trigger mechanism are subject to wear and tear during operations. Once the trigger fails to work properly, the flywheel (a) may reach abnormally high speed, causing accumulated energy to increase beyond the design limitation which may lead to serious damages to the impact pair.
Another disadvantage of traditional energy-accumulating pneumatic wrench is its rather small retracting force for impact pin (c). The air pressure applies to the whole surface of the piston to push the impact pin (c) out, but only to the annular surface of the piston to retract the impact pin (c). During operation, the impact pin (c) is always stuck after a “soft impact” due to friction.
These problems can be solved effectively with the wholly air-controlled impact mechanism presented herein.